WASHINGTON (CNN) - As one so-called White House czar resigned over the weekend, President Obama announced the appointment of another one Monday, much to the frustration of Republican critics.

By some accounts, Obama has nearly 30 czars, who are officially called special advisers. The czars cover issues from AIDS and health care to Middle East peace.

Czars are nothing new. They date back to early presidents, including Franklin Roosevelt. Republicans also had czars: Richard Nixon had an energy czar, and George H.W. Bush appointed the first drug czar.

But the positions are not subject to congressional oversight or Senate confirmation, which rankles critics of the administration.

"What you see with President Obama is this reliance on czars," GOP strategist Kevin Madden said. "And I think there are probably even some corridors of power within the administration that probably didn't like the idea that you have czars that are encroaching on their policy portfolios."

David Gergen, a CNN contributor and former aide to past presidents, said the czar controversy has given Republicans an opening to question the administration's decisions.